The present invention relates generally to a semiconductor optical device, and more particularly, to an external modulator integrated laser which operates at a transmission rate of 2.5 Gbits/s or higher, and an optical transmission module having the external modulator integrated laser mounted thereon.
At present, dense wavelength division multiplexing (DWDM) optical transmission which transmits optical signals of multiple wavelengths in a single optical fiber most typically uses a transmission system which enables long-range transmission at a transmission rate of 2.5 Gbits/s or higher, and in particular, 10 Gbits/s. Market demand for a transmission rate of 40 Gbits/s has recently been gradually increasing.
In the DWDM optical transmission, a grid wavelength with a wavelength band of 1.55 μm that realizes long-range optical transmission is defined in the ITU-T standards. The wavelength interval (frequency interval) of the grid wavelength is 0.4 nm (50 GHz) or 0.8 nm (100 GHz). It is therefore crucial that the oscillation wavelength of a semiconductor optical device to be used in such optical transmission fluctuates with the passage of time.
Light sources to be used in DWDM optical transmission include a small-sized, low-cost electro-absorption (EA) modulator distributed feedback (DFB) integrated laser (hereinafter called “EA/DFB integrated laser”). Particularly, a buried heterostructure (BH) having an optical waveguide structure, which forms an EA/DFB integrated laser and is planarized by burying a mesa with a semi-insulative semiconductor, such as iron doped indium phosphorus (Fe-InP), is typically used in DWDM optical transmission, since the oscillation wavelength of the laser portion is very stable.
A process of forming a BH using an n-type InP substrate will be briefly described below referring to FIGS. 6A to 6D and FIGS. 7E to 7G.
First, oxide films (SiO2) 102 are formed on an n-type semiconductor substrate 101 by T-CVD (Thermal-Chemical Vapor Deposition), and the SiO2 films 102 sandwiching that region of a laser portion 103 equivalent to a mesa are patterned. With the SiO2 films 102 as masks, an n-type InP buffer layer 104, an n-type InGaAsP lower guide layer 105, a multi-quantum well (MQW) 106 having a well/barrier layer of InGaAsP, a p-type InGaAsP upper guide layer 107, and a p-type InP cap layer 108 are grown in order by selective area growth method called MO-CVD (Metal-Organic Chemical Vapor Deposition) (first multilayer growth: FIG. 6A)
With the presence of the SiO2 masks in the laser portion 103, the selective growth effect makes the laser portion 103 thicker than an EA modulator portion 109. Further, changing the interval between the SiO2 masks can change the photoluminescence wavelengths of the laser portion 103 and the EA modulator portion 109, which affect the characteristic of the EA/DFB integrated laser.
Thereafter, the SiO2 masks 102 are removed, and only the p-type InP cap layer 108 of the laser portion 103 is etched to form a grating 110 in a part of the upper guide layer 107 by interference exposure (FIG. 6B).
Next, a p-type InP clad layer 111, a contact layer 112 of p-type InGaAsP and p-type InGaAs, and a p-type InP protection layer 113 are regrown by MO-CVD (second multilayer growth: FIG. 6C). The first multilayer and the second multilayer both generally use zinc (Zn) as a dopant for p-type InGaAsP, p-type InGaAs and p-type InP. Next, the p-type InP protection layer 113 is removed, and with an SiO2 film 114 as a mask, mesas are formed in the laser portion 103 and the EA modulator portion 109 to a sufficient depth (e.g., 3 μm or so) from the position of the MQW 106 and with a width of 1 to 2 μm by dry etching or wet etching (FIG. 6D).
Thereafter, buried growth of an Fe-InP semi-insulative semiconductor 115 in the other regions than the mesas is carried out again, and then the SiO2 film 114 is removed to form a BH (FIG. 7E).
Subsequently, the contact layer 112 directly above the mesa between the laser portion 103 and the EA modulator portion 109 is etched off, thereby forming an isolation groove to electrically isolate the laser portion 103 from the EA modulator portion 109. Further, the entire wafer is protected with a passivation film 116 by T-CVD. Thereafter, the passivation film 116 on the mesa regions of the laser portion 103 and the EA modulator portion 109 is removed to form a through hole. Further, a p-side electrode 117 of Ti/Pt/Au is vapor-deposited, and is patterned by ion-milling. Thereafter, the resultant wafer is polished until the wafer thickness becomes 100 to 150 μm, and an n-side electrode 118 of AuGe/Ni/Ti/Pt/Au is vapor-deposited to complete the wafer (FIG. 7F).
The wafer is cleaved into bars, the cleave facet of the device's rear side (laser side) is protected with a high reflection film 119 having a reflectance of 90% or greater, and the cleave facet of the device's front side (EA modulator side) is protected with a non-reflection film 120 having a reflectance of 1% or less, and finally the wafer is cleaved to chips (FIG. 7G).
FIG. 7G shows the cross-sections of the laser portion 103 and the EA modulator portion 109 of the EA/DFB integrated laser completed this way (cross-sectional view along B-B′ and cross-sectional view along C-C′).
To check the stability of the oscillation wavelength of the thus configured EA/DFB integrated laser with a BH, a temperature cycle test to intentionally apply a load to the laser was conducted as follows.
In the temperature cycle test, 50 or more cycles each consisting of a process of placing the EA/DFB integrated laser into a thermostatic chamber of a nitrogen atmosphere and increasing the environment temperature from −40° C. to +85° C. at a rate of about 10° C./minute, and a process of then cooling the device to −40° C. (or cooling the laser from +85° C. to −40° C., then increasing the temperature to +85° C.) are repeated. In the test, the oscillation wavelengths of eight devices were checked in around 150 cycles.
The test results showed that the wavelength fluctuation was very small (−9 to 0 pm) and the laser with the BH would be suitable for DWDM optical transmission.
As mentioned above, recently the market demand for 40-Gbits/s DWDM optical transmission is increasing gradually. To ensure a fast operation, the modulation band of the EA modulator needs to be made sufficiently large (e.g., the modulation band is desirably 35 GHz or greater in cases of operation at 40 Gbits/s). To achieve fast operation, the parasitic capacitance of the EA modulator portion should be made small, to about 0.1 to 0.2 pF.
It is known that in the BH of Fe-InP, Zn as a dopant for the p-type InP clad is likely to be interdiffused with Fe under a high temperature of around 600° C. at the time of buried growth. Therefore, the diffused Zn undesirably produces an uncontrollable parasitic capacitance or causes a sudden increase in capacitance in the mesa side, thereby adversely affecting the modulation band of the EA modulator. Recently, “R. Iga et al. ‘Ru-doped Semi-Insulating Buried Heterostructure Laser Operating up to 100° C. for 10-Gbit/s Direct Modulation’, ECOC 2005, Tu 4.5.2” has reported that a mesa side is buried with a semi-insulating semiconductor of ruthenium-doped indium phosphorus (Ru-InP). While the use of Ru has such an advantage as to make the capacitance of the EA modulator portion easily controllable, since Ru is hardly interdiffused with Zn, it is difficult to stably provide the material. It is therefore difficult to mass-produce semiconductor optical devices to which a Ru-InP buried structure is applied.
In cases of a ridge waveguide structure using an n-type InP semiconductor substrate, after a mesa is formed, the mesa side is buried with a dielectric substance of a low dielectric constant (about 1.5), such as a resin. At this time, because the substrate is not exposed to a very high temperature as done for the BH, interdiffusion which increases the capacitance does not occur. It is therefore possible to make the capacitance sufficiently small and it is easier to design the capacitor itself. It is thus very effective to apply the resin-buried ridge waveguide structure to the EA modulator or the like which operates at, for example, 40 Gbits/s.
A description will now be given of a fluctuation in the oscillation wavelength of the EA/DFB integrated laser in a case where, like the mesa structure of the EA modulator portion, the mesa structure of the laser portion employs a resin-buried ridge waveguide structure to make the fabrication of the EA/DFB integrated laser simpler. A temperature cycle test of 150 cycles or so was conducted on ten EA/DFB integrated lasers whose EA modulator portion and laser portion both have a resin-buried ridge waveguide structure, as done on an EA/DFB integrated laser with a BH. The test results showed a very large fluctuation in oscillation wavelength of −24 to +20 pm.
While this amount of wavelength fluctuation does not matter for a semiconductor optical device to be used in time division multiplexing (TDM) optical transmission, it is not preferable for a semiconductor optical device to be used in DWDM optical transmission in terms of reliability. The wavelength fluctuation seems to have originated from a very large stress applied to the mesa region according to a change in temperature because the difference in the coefficients of thermal expansion of the semiconductor forming the mesa and the resin burying the mesa side is large (several ppm/° C. v.s. for the semiconductor, and about 50 ppm/° C. for the resin). In the case of the BH, however, the mesa and the mesa side are of the same semiconductor material, and stress is hardly applied to the mesa, so that the oscillation wavelength is very stable.
The laser portion may have a BH and the modulator portion may have a ridge waveguide structure (FIG. 8) as described in “C. Rolland et al., ‘InGaAsP-based Mach-Zehnder modulator for high-speed transmission systems’, OFC '98 ThH1”. Although the mesa side of the modulator portion is air in this document, the mesa width is the same as the width of the p-side electrode, so that no extra capacitance other than the PIN junction capacitance is produced in the mesa side. This achieves a low capacitance. Because the laser portion takes a semiconductor-buried BH, stress is not applied to the mesa, so that the oscillation wavelength is stable. When the material for the MQW is InGaAlAs, however, the MQW in the mesa side wall contacts air after the mesa is formed, so that the Al-based material, InGaAlAs, is likely to be oxidized as compared with the P-based material, InGaAsP, described in the document. It is necessary to carefully perform a pre-process thereafter to remove Al oxide immediately before forming the BH. If the BH is formed in the laser portion with an impurity, such as Al oxide, adhered to the mesa side wall, the impurity becomes a leak path or the like, for the laser injection current, thereby adversely affecting the reliability of the laser. Therefore, the use of an Al-containing material makes it very difficult to form a BH in the laser portion, and in actuality may cause a sudden fault.